Root Morphology and Anatomy Affect Cadmium Translocation and Accumulation in Rice

Paddy fields contaminated with cadmium (Cd) present decreased grain yield and produce Cd-contaminated grains. Screening for low-Cd-accumulating cultivars is a useful method to reduce the amount of Cd in the grains. The present study aimed to examine the roles of the root morphology and anatomy in Cd translocation and accumulation in rice plants. Twenty-two rice cultivars were used in the first experiment, after which two cultivars [Zixiangnuo (ZXN) and Jinyou T36 (JYT36)] were selected and used in subsequent experiments under hydroponic conditions. The results showed that there were significant differences in Cd concentrations in the shoots (ranging from 4 to 100 mg/kg) and the Cd translocation rates (shoot/root) (from 7% to 102%) among the 22 cultivars, and the shoot Cd concentration was significantly correlated with the Cd translocation rate of the 22 cultivars under 0.1 mg/L Cd treatment. Compared with cultivar ZXN, JYT36 had greater root Cd uptake and accumulation but lower shoot Cd accumulation and Cd translocation rate. The number of root tips per surface area of cultivar ZXN was greater than that of JYT36, while the average root diameter was lower than that of JYT36. Compared with ZXN, JYT36 had stronger apoplastic barriers, and the Casparian bands and suberin lamellae in the root endodermis and exodermis were closer to the root apex in both the control and Cd treatments, especially for suberin lamellae in the root exodermis with Cd treatments, with a difference of 25 mm. The results also showed that, compared with ZXN, JYT36 had greater percentages of Cd bound in cell walls and intracellular Cd but lower Cd concentrations in the apoplastic fluid under the Cd treatment. The results suggested that Cd translocation, rather than root Cd uptake, is a key process that determines Cd accumulation in the rice shoots. The root morphological and anatomical characteristics evidently affect Cd accumulation in the shoots by inhibiting Cd translocation, especially via the apoplastic pathway. It was possible to pre-screen low-Cd-accumulating rice cultivars on the basis of their root morphology, anatomical characteristics and Cd translocation rate at the seedling stage.